The present invention relates to a video interface system provided between an image forming apparatus, such as a digital copy machine used as a printer, and an image data supplying apparatus, such as a host computer, which sends image data to the image forming apparatus.
Conventional video interface systems include two kinds of systems. One system is the so-called fixed frequency system in which only image data is transferred from a host computer to a printer used as an image forming apparatus, or image data and an internal clock signal are transferred together from the host computer to the printer. The other system is a system in which a clock signal is first supplied from the printer to the host computer, and then the host computer transfers image data in synchronization with the clock signal to the printer.
In the fixed frequency system, it is difficult to use with a host computer a plurality of printers having different clock frequencies since the clock frequency of the host computer is fixed. In this system, it is desirable that the host computer change the clock signal depending on the printer to be connected thereto. However, it is difficult to change the clock signal to correspond to a wide range of frequencies, and if there appears a printer having a new clock frequency, the host computer cannot cope with the new frequency.
Since the clock frequency used in the printer generally depends on the print speed, it is difficult to unify the clock frequency for the different types of printers. Even though the clock signal of the printers can be unified using a page buffer for each printer, the manufacturing cost of each printer is then increased.
In the latter system, since the clock frequency can be determined on the printer side, the above-mentioned problems can be eliminated. However, there is skew between the image data and the clock signal, which skew is caused by a delay in a transmission path, and thus this system is applicable only for a video interface having a low data transmission rate and a short transmission path.